Non-H98 crusade, contd.
k.schupke at imperial.ac.uk
Mon Feb 28 06:25:26 EST 2005
ross at soi.city.ac.uk wrote:
>On Sat, Feb 26, 2005 at 08:00:22PM -0500, ajb at spamcop.net wrote:
>>Quoting ross at soi.city.ac.uk:
>>>Indeed. Of all the extensions implemented by both GHC and Hugs, the only
>>>ones that seem ready are
>>>- rank 2 type signatures, and
>>>- polymorphic components for data constructors (giving them rank 2 types).
>>Off the top of my head:
>>- multi-parameter type classes
>Reasonable in themselves, but limited in usefulness without some scheme to
>deal with overlapping instances, which doesn't seem settled at this time.
In the HList paper we show that any overlapping instance can be
eliminated with the use of a single constraint (that would have to be
provided by the compiler: TypeEq).
Further I think functional dependancies (without overlapping instances),
and undecidable instances are perfectly well understood. (incoherent
instances would join overlapping instances and not be included).
So I would add:
- Multi-parameter type classes with functional dependancies (with no
overlapping instances allowed)
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